High-frequency heating



VOLTAGE Nov. 20, 1951 w. s. THOMPSON 2,575,604

HIGH-FREQUENCY HEATING Filed Sept. 15, 1947 FRE QUENCY MODULATED OSCILLATOR FRE QUE NCY MODUL ATED OSCILLAT //vl/6/\/70 q WALT m S. THOMPSON MM /MW AT TULZ/VE /S Patented Nov. 20, 1951 UNITED STATES PATENT OFFICE HIGH-FREQUENCY HEATING Walter S. Thompson, Portland, Oreg., assignor to M and M Wood Working Company, Portland, reg., a corporation of Oregon Application September 15, 1947, Serial No. 774,045

3 Claims. 1

This invention relates to improvements in high frequency electrical heating and ha particular reference to the heating of dielectric bodies of large dimensions.

In heating dielectric bodies by high frequency alternating currents, the electrodes are arranged to produce a dielectric field including the body to be heated. If uniform heating of the body is desired, an electrode arrangement must be employed which will develop a reasonably uniform dielectric field in all parts of the body to be heated, and which is adaptable to the physical limitations of the apparatus with which it must be used. In heating thermosetting glue in a plywood press or scarf joint press, for example, it is convenient to utilize the metallic press platens as electrodes. When the length of each high frequency conductor from its coupling to the most remote extremity of its electrode is short in relation to the wave length of the energy radiation, satisfactory uniform heating of the glue may be accomplished.

However, with the use of larger presses and higher frequencies, difficulty has been experienced in obtaining uniform heating in all parts of the work in the press. It is found that variations in heating in different parts of the dielectric material are caused by the existence of different potentials at different points on the electrodes. When the conductor length is greater than a quarter Wave length the standing wave produced has one or more fixed nodal points. t such nodes the potential on the conductor remains a minimum at all times and the heating effect in the dielectric adjacent such points is likewise small. Thus the occurrence of a standing wave produce uneven heating, and if one of the nodal points falls in the electrode very little heat is produced in the work at such point.

If the heating interval is sufficiently long, heat flow through the work will tend to equalize the temperature from point to point, but ordinarily the heat deficiency in a cold spot cannot be supplied in this way because the time is too short and most dielectric bodies are poor heat conductors. The principal advantage of high frequency heating are speed and economy of energy and these would both be lost if the heating interval were prolonged sufficiently to heat the cold spots by heat conduction from the hotter zones. Also, the temperature of the hot zones might then need t be so high as to damage the wood during an extended heating interval.

The general object of the present invention is therefore to provide an improved method of high frequency heating to overcome the disadvantages pointed out and accomplish substantially uniform heating of a dielectric material.

Another object is to provide a method of heating dielectric bodies in a high frequency field to produce a substantially uniform heating effect regardless of the size of the body or the frequency of the voltage wave.

Another object is to provide a method of and means for preventing permanent fixed nodal points in standing waves in high frequency heating circuits.

A specific object is to utilize frequency modulation to shift the position of standing waves in a high frequency heating circuit.

In carrying out the objects of the invention the occurrence of cold spots at fixed points in the dielectric body resulting from a standing wave is prevented by rapidly shifting the position of the voltage nodal points with respect to the dielectric body. This is conveniently accomplished by frequency modulation of the voltage wave. As the frequency changes, the wave length changes and the nodal points move accordingly so that no point on the conductor is continuously at minimum potential. Inasmuch as the heating effect in the dielectric body varies with voltage intensity from point to point, the variations in heating effect are thereby attenuated to achieve a satisfactory degree of uniformity of heating throughout the whole body. For reasons which will become apparent as the description proceeds in connection with the accompanying drawings it is preferred t make the average frequency sufficiently high to produce a number of complete cycles within the work. It is also desirable to make the modulating frequency sufiiciently high so that the time of one cycle is short with respect to the length of the heating interval.

In the drawings:

Figure l is a schematic representation of apparatus for practicing the method of the invention in connection with the gluing of plywood and the like in a press having a pair of insulated electrodes to produce a single dielectric circuit through the work;

Figure 2 is a view similar to Figure 1 showing how the invention may be applied to a conventional press having grounded platens by inserting an intermediate electrode in the press load to establish a pair of dielectric circuits through the Work;

Figure 3 is a diagram showing the effect of frequency modulation on a standing Wave in a high frequency circuit; and

Figure 4 is a diagram showing the beneficial effect of frequency modulation in attenuating the variations in heating effects produced by a standing voltage wave in the electrodes shown in Figures 1 and 2.

In Figure l the numerals I and 2 designate mutually insulated electrodes of opposite polarity on opposite sides of a dielectric body 3 to be heated. These electrodes are connected by conductors and 5 with a high frequency oscillator which is herein designated as a frequency modulated oscillator in order to carry out the principles of the invention.

Figure 2 illustrates the application of high frequency heating to a conventional press .ofzthe.

type used for making plywood or for .making scarf joints and the like wherein the upper and lower press platens I and 8 are utilized as electrodes on opposite sides of a press load of -panels a. Such platens are ordinarily grounded to the frame whereby they are electrically connected together as indicated by the wire 4a. The panels in the stack 3 contain a thermosetting glue which may be eificiently heated in a high frequency field to raise the temperature of the glue to cause it to set permanently and form a bond with the wood veneers or the plywoodasthe case maybe. The press load 3 is thereby made up of two dielectric materials,one being the wood and the other being the glue filmsin the joints to be glued, It is desired'to heat theglue; films as uniformly as possible so that all the glue Will be heated to its critical temperature without heating some of the glue to its temperature-ofdecomposition and without causing, damage tothe wood,

It is also desirable toaccomplish the heat setting of the, glue as quickly as possible to increase the output of the press and to reduce to aminimum the time of exposure of the woodto elevated drying temperatures, The entire heating, interval,

is therefore not of sufficient durationto accom- .plish any effective attenuation ,of temperature variations inthe glue in .diiferentparts ofthe middle of thestack 3 as shown, this electrodethen being connected to the wire 5 fromtheoscillator. Two separate dielectric fields are thereby .produced between the pressiplatens I :and 8, but in the subsequent analysis .of. the heating. action only one of these fields will be. considered as would bethe case in Figure 1.

V In Figure Bthe numeral I designates a coupling with a high frequency oscillator,;having a pair of conductors l l and I2 extending therefrom, these parts being shown in phantom. If Figure 3 is referred, for example, to Figure 1, theconductor H would comprise the wire 4 andtheelectrode I extending to the length shown from-the coupling Ill. Similarly the conductor. ,comprises the wire and electrode}. (The vvertical line. I3 is drawn through the, coupling l I). to form with the line representing, the :conductor I2. the

. coordinate axes of a graph onwhichis. drawn 7 a voltage curve Moi a standing wave in theiout- I put of the system. The superposition ofthevoltage curve M upon the phantom representation of-theconductor I2 thereby illustratesthe variations in potentialfrom. point to .point produced by the standing wave in theconductor. The con-'1 J. standing wave then has a maximum value on one side of the press.

side of thepress and minimum value on the other With a fixed frequency, no

xamountgofincrease in the output would be effec- :tive in heating the low potential side of the press,

' lexceptby heattransfer from the hot side.

. In the example illustrated in Figure 3, the wave length is approximately equal to the width of the .press platens, thereby developing maximum voltage in the standing wave at points l5, I6 and I1 and minimum voltage at thenodal points I8 and I9. I The heating curve for such a standing wave is illustrated by the line I ia in Figure 4. Here the electrode or platen 2 is shown in phantom, and only that part of the circuit having an immediate effect upon the body to be heated is illustrated. Thus, the points of maximum and minimum heating effect are'designated by the same reference characters used on the voltage pronounced because it will influence. a smaller area or volume of the body to be heated, but such points I nevertheless represent cold spots into which the heat cannot flow fast enough to raise the temperature to the required critical value in the time allowed without greatly'overheating the hot spots I5a, ISa and Ila.

The present solution to the problem-is thereforeto shift the nodal points back and forth on the electrode so that within the heating interval heat will be applied with more uniformity to all parts of; the dielectric body. Since the point I 5 cannot beshifted without mechanically changing I, the length of the conductor it is preferred to shift the q al sb cha in the wavelen th which, is easily accomplished by--providing the oscillator with frequency modulation.

If-zthe frequency is doubledthe standing voltage curve appears as the broken :line curve v2!] '25,, 26, 21, 2B and :29. iiWith, a shift to/ double frequency, the heating loopsandnodes aremoved having maxima at I5, v2I,=';i2, :23, vl'i',..ancl"24. Nodes of minimum voltage. are then, formed at as shown by the broken line curve 20a infligure 4. The nodeylila ismovedtothe point Zii, .thenode -I 9a ,is .moved .to .the point 26, .etc. Likewise the former node I9a. Thus it is seen-that if the fre- The modulating-action; of course,

. moves'the loops and nodes progressively and continuously. between certain predetermined limits depending upon the extent of movement desired.

For a ratio of frequency limits of 1 to 2 the curves I 4 and lfia-may be considered asrepresenting the lower frequency limit and the curves 20 and 20a. as representing the upper frequency limit. These frequencies produce one wave length across plate electrodes of the length shown in Figures 1 and 2 at the lower frequency limit and two wave lengths across the electrodes at the higher frequency limit, the average frequency producing one and one-half Wave lengths.

However, the curves shown in Figures 3 and 4 are merely diagrammatic and do not represent the actual number of wave lengths desired across the width of the platen. As previously mentioned, it is desired in practice to employ a frequency range having an average value to produce a number, such as ten or more, wave lengths across the width of the body to be heated instead of only one and one-half wave lengths as shown in Figure 3 for convenience of illustration. The number of wave lengths across the body is of course not critical as long as this condition is satisfied. Any degree of modulation of this average wave length is helpful, but, as explained above, the modulation is most efiective if the upper and lower limit frequencies have at least a two to one ratio. The modulation frequency has no critical value except as previously stated that the time of one modulation cycle should be short in relation to the duration of the heat treatment period, in order to shift the nodal points repeatedly during the treatment.

The invention is, of course, not limited to the heating of glue nor to applications involving press platens. It is of general application to all forms of dielectric field heating where it is desired to employ electrical frequencies high enough to produce standing waves in the electrodes employed.

Having now described my invention and in what manner the same may be used, what I claim as new and desire to protect by Letters Patent is:

1. In high frequency electrical energy heating employing electrodes and conductors having a length which is appreciable compared to the wave length of said energy and normally subject to a standing wave having a nodal point in said electrodes whereby the heatin effect varies along said electrodes in accordance with the position of said wave, the method of attenuating variations in said heating effect in a heating period comprising continuously varying the frequency of the source of said energy in a modulating cycle which is short in relation to the heating period 6 and between limits in which the highest frequency is approximately double the lowest frequency to place wave crests at the highest frequency repeatedly in positions occupied by the nodal points at the lowest frequency.

2. The method of heating dielectric material between electrodes in a heating period by the use of high frequency electrical energy where the electrodes have a length which is appreciable compared to the wave length of said energy and normally subject to standing waves having at least one nodal point in the electrodes, comprising applying a high frequency potential to said elec trodes to produce said standing wave, and varying the frequency of the source of said potential rapidly in relation to the length of the heating period and sufficiently to move the crests and nodes of said wave back and forth to cause a. nodal point and wave crest to fall repeatedly at the same point in said electrodes at different .times in a heating period, to attenuate variations in the heating effect produced by the standing wave.

In a high frequency electrical energy heating system having electrodes of a length which is appreciable compared to the wave length of the energy employed and normally subject to standing waves having at least one nodal point in said electrodes at which a minimum heating effect is produced, a frequency modulated oscillator for producing said standing wave, the frequency of said oscillator being controlled to vary the frequency between limits having the approximate ratio of 1 to 2 to place the nodal points of the wave at the lowest frequency in positions occupied by wave crests at the highest frequency.

WALTER S. THOMPSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,226,871 Nicholas l Dec. 31, 1940 2,307,344 Zottu Jan. 5, 1943 2,308,043 Bierwirth Jan. 12, 1943 2,381,496 Hansell Aug. 7, 1945 2,449,451 Cassen Sept. 14, 1948 2,483,569 Baker Oct. 4, 1949 2,537,193 Shaw Jan. 9, 1951 

